The present invention relates to optical fibers for use in light transmission wherein the range of spectral bandwidth of the light source is narrow and the power of the light injected to the optical fibers is large; these optical fibers can prevent the occurrence of the induced Brillouin scattering.
In accordance with the advance of optical technology, a high energy light source is easily obtainable through an optical amplification method, this light source also has a narrow spectral bandwidth of less than several 10 MHz. An optical fiber having a transmission-loss ratio which almost equals the theoretical ratio, i.e., 0.20 dB/km, is also producible in a mass production. As a result of these improvements, an unrepeated transmission distance can be increased up to about 300 km. As an optical fiber for use in a transmission network system, a single mode optical fiber is employed.
However, when strengthening the intensity of the input light to the optical fiber in order to further lengthen the transmission distance, when the intensity of the light is greater than a certain threshold value, non-linear optical effects occur, such as Laman scattering, induced Brillouin scattering, and Four-Photon Mixing. In these effects, the induced Brillouin scattering and the Four-Photon mixing can be observed in a relatively low intensity range of input light. By using a coherent light of the source which has a smaller spectral bandwidth of about several hundred MHz, which corresponds to Brillouin's bandwidth, an induced Brillouin scattering can be easily observed.
The induced Brillouin scattering is a phenomenon in which light is scattered by a slight deviation of frequency which is caused by a longitudinal acoustic wave in the glass constituting the optical fiber. The light is scattered in a direction in which the phase of the incident rays and the scattered light coincide. On the other hand, the natural frequency of the acoustic wave in the optical fiber is in the range of several GHz to several tens of GHz, and the transmission velocity thereof is about several thousand m/min. Therefore, the direction in which the interaction between the incident light and the acoustic wave is maximum is the direction opposite to that of incident light; the result is that the scattered light returns from the inside of the optical fiber to the incident end thereof.
FIG. 4 illustrates a measuring system for measuring the amount of light scattered by induced Brillouin scattering. The measuring system is comprised of a signal source 4, an optical amplifier 5, and an optical fiber loop 7 connected to one another via a branching optical coupler 6. By this construction, the measuring system can measure the amount of light generated by induced Brillouin scattering, which is scattered in a backward direction from the optical fiber loop 7 and is branched by the branching optical coupler 6. In the FIG. 4, a transmission of signal light is indicated by continuous lines and a transmission of light of induced Brillouin scattering is indicated by chained lines. The signal light is injected from the signal source 4 into the measuring system and amplified by the optical amplifier 5, is further branched by the branching optical coupler 6. Thus, a small part of the incident light is guided to an optical power meter 1 and detected thereby, and the larger part of the incident light is guided to the optical fiber loop 7 and injected thereto. In the optical fiber loop 7, the incident light generates Brillouin scattering, and the light transmitted through the optical fiber loop 7 in the forward direction is injected into the optical power meter 2 and is detected thereby. The light scattered in the backward direction is guided to the optical power meter 3 via the optical coupler 6 and is detected thereby.
By using the measuring system having this construction, the intensity of Brillouin scattering was measured around the conventional optical fiber. The construction parameters of the optical fiber are shown below. The optical fiber is connected to the measuring system, and the quantity of both the transmission light detected by the optical power meter 2 and the scattered light detected by the optical power meter 3 are respectively measured while the supply of the incident light is altered. The optical coupler 6 splits the injected light at a constant ratio so that the quantity of incident light can be estimated by the quantity measured by the optical power meter 1.